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The Best of Both Worlds? Fertigation Is
an Efficient Way for Many Farmers To Grow Crops

A fertigation system used on romaine lettuce in Coachella
Valley, California. Metered chemical fertilizers are introduced
through plastic tubing and injection ports at various points in the
irrigation pipe. An elbow provides mixing of the injected
chemical with the irrigation water, which is then distributed into
multiple furrows. (K11613-1)

Many growers don't simply water their crops. They "fertigate"
them. As the name implies, fertigation brings both nutrients and water to
plants.

Fertigation has many advantages over applying water and
fertilizer separately. It saves money by combining the two tasks. It allows
growers to fertilize crops throughout the growing season rather than stop when
the plants become too unwieldy to allow mechanized applications with
conventional machinery. Finally, many crops can thrive with less fertilizer
when it's applied throughfertigation.

Farmers are interested not only in increasing crop yields, but
also in improving water quality, according to Dale A. Bucks,
ARS national program leader for water
quality and management. Fertigation began with sprinkler irrigation but has
advanced to surface and other irrigation systems. Surface irrigation has the
added benefit of curbing both nutrient runoff and leaching into streams and
groundwater. As soil scientist Floyd Adamsen of ARS's U.S. Water Conservation
Laboratory in Phoenix, Arizona, points out, "With fertigation, we're trying to
get the fertilizer to stay where we want it."

This high-clearance sprayer makes variable-rate nitrogen
applications to corn based on sensor readings. The sensors
monitor plant stresses that are frequently related to nitrogen status.
(K11575-1)

The main variable Adamsen and agricultural engineer Douglas J.
Hunsaker studied was the timing of fertilizer injections throughout an
irrigation cycle on date palms grown in borders on a sandy soil in California's
Coachella Valley. They found, using bromide as a tracer and stand-in for
fertilizer, that it was best to inject fertilizer during the entire irrigation
process. The researchers also looked at adding bromide to irrigation water only
during certain segments of the irrigation, but this was less successful. The
results were similar to studies conducted on furrow irrigation systems in
Arizona.

The problem with current fertigation practices for
surface-irrigated fields is that the mixture isn't always distributed
uniformly. Water draining off the field can carry fertilizer with it. Improved
surface fertigation practices can reduce field variation to only 10-15 percent.
Fertigation performance was improved by ensuring that water applications were
optimized for the field and crop conditions and by precise control of
fertilizer injections.

These Crimson Lady peach trees irrigated by subsurface
drip outgrew trees irrigated by other methods during the first 3 years
after planting. (K11614-1)

Newer, pressurized irrigation systems have less field
variability of water and fertilizer than surface irrigation does. But many
growers are not yet willing to replace surface irrigation with the newer
systems. So many areas of the West and Southwest stick to fertigation through
surface irrigation. Most wheat growers and a third of cotton growers in Arizona
fertigate through surface irrigation systems, according to Steven Husman, a
University of Arizona extension agent.

Sensing the Needs

You might say Jim Schepers, Dennis Francis, Mike Schlemmer, and
Ariovaldo Luchiari are using a short form of remote sensing in their
fertigation studies in Lincoln, Nebraska. Instead of using aerial photographs
or satellite-generated images, the scientists take stock of the crop's
fertilizer needs from a few feet off the ground. Their tool of choice:
electronic sensors perched atop high-clearance canopy sprayers.

The sensors zap the crop with certain light wavelengths and then
measure how much of that light bounces off the plant's surface. The brightness
of the returning light is then assigned a numeric value.

One way the sensors check for onset of stress due to nitrogen
deficiency is to zap the crop with red light, which is absorbed by chlorophyll,
an important pigment. Since green, healthy plants have lots of chlorophyll in
their leaves, they absorb more red light, and reflect less, than
low-chlorophyll plants, such as those needing nitrogen fertilizer. Near
infrared, a waveband not visible to humans, is sensitive to the amount of
living vegetation present, so it is used to assess plant vigor.

The high-clearance sprayers use such information to fertigate
with variable, rather than fixed, rates of nitrogen, saving money and reducing
the risk of leaching. The sprayers' sensors can monitor the conditions of
multiple plants, rows, or areas and collect reflectance readings from them at
the rate of 1 to 10 per second. "This allows us to make spatial nitrogen
applications that simulate variable-rate fertigation," says Schepers, research
leader of ARS's Soil and Water Conservation Research Unit at Lincoln.

The sensor studies are part of a cooperative multistate project
under way to evaluate this type of fertilization technology for use on corn,
wheat, andto a lesser degreeturf grass. Other ARS scientists in
Fort Collins, Colorado, are applying fertilizers and pesticides using precision
agriculture technologies, including variable-rate fertigation (see
Agricultural Research,
October 2000).

Schepers says the technology may eventually allow farmers to
satisfy their crop's nitrogen fertilizer needs in season rather than trying to
predictbefore plantingwhat these will be. Aided by technologies
such as the sensors, "Our strategy is to make sure the crop gets off to a good
start, monitor its progress, and provide required nutrients as needed."

Secrets From Underground

Meanwhile, scientists are scrutinizing healthy young peach trees
to find out how to fertigate for the best yields of this delicious fruit.
Growers already know a lot about how to manage a mature peach orchard. But
there's very little scientific information on the water and nutrient needs of a
newly planted orchard.

There's ongoing interest in the care of young trees. That's
because many commercial peach orchards are pulled up every 10 years or so and
replaced with new varieties that have more economic potential, says plant
physiologist David R. Bryla. Formerly with ARS's Water Management Research
Laboratory, Parlier, California, and now with ARS in Corvallis, Oregon, Bryla
leads the peach study. He expects to have final results later this year.

Earlier Parlier investigations of fertilizer and water needs of
three other cropssweet corn, cotton, and tomatoesare attracting
renewed interest. Completed by ARS scientists about a decade ago, the research
demonstrated the benefits of subsurface drip systems. Through tubes buried
beneath the soil, these systems bring precise amounts of water and fertilizer
to the place they're needed mostplants' roots.

James E. Ayars, agricultural engineer at the water management
lab, says several factors combined to boost interest in subsurface systems,
including the need to prevent seepage of excess nutrients into groundwater and
new, impressive improvements in subsurface drip equipment.

The Parlier scientists' detailed studies of crops' water use and
yield with subsurface drip, as compared to surface systems, still rank as the
most comprehensive of their kind.By David Elstein,Jan Suszkiw, and
Marcia
Wood, Agricultural Research Service Information Staff.

This research is part of Water Quality and Management, an ARS
National Program (#201) described on the World Wide Web at
www.nps.ars.usda.gov.